This invention relates to optical fiber light sources as well as to optical fiber amplifiers (FAMPs).
In conventional FAMPs, signal light to be amplified propagates through a section of fiber that is doped to provide optical gain over a range of wavelengths. Gain is induced by coupling suitable pump light from a pump laser into the gain section while the signal light is propagating therethrough. The pump light may be co-propagating or counter-propagating, or both, depending on system considerations. Since the signal light (.lambda..sub.s) and the pump light (.lambda..sub.p) have different wavelengths, the coupling is advantageously done by means of a wavelength division multiplexer (WDM).
In one prior art design the pump laser is a diode laser which is coupled through a single mode fiber and the WDM to an Er-doped fiber. For FAMP operation at about .lambda..sub.s =1550 nm the diode laser illustratively generates pump light at 980 nm. In other designs the pump laser is a fiber laser, which itself is energized by a diode laser, and is likewise coupled through a single mode fiber and the WDM to an ER-Yb-doped fiber. For FAMP operation at about .lambda..sub.s =1550 nm the fiber laser generates pump light at 1060 nm.
In the remainder of this description, we will concentrate primarily on FAMPs which are pumped by fiber lasers. In a typical design of a fiber laser, an active fiber medium is interposed between a pair of fiber gratings which form a resonator. Energizing light from a diode laser is coupled, for example, through one of the gratings (input grating) into the active medium; pump light generated in the active medium finds egress through the other grating (output grating).
The active medium of the fiber laser, typically a fiber section doped with Yb or Nd, is capable of lasing over a relatively broad band. Lasing at a particular wavelength (e.g., 1060 nm) that even low level (e.g., 4%) reflections may be sufficient to induce lasing at these undesired wavelengths. Such low level reflections can occur, for example, at the cleaved end face of the fiber section to which the diode laser is coupled.
Thus, there is a need in the fiber laser art to prevent such lasing at undesired wavelengths.
This FAMP design also suffers from another problem which is related to the reliability of the diode laser used to energize the fiber laser. More specifically, although the input grating of the fiber laser is nominally 100% reflective at the fiber laser wavelength, it is less than perfect. Therefore, a portion of the fiber laser light is incident on the diode laser. If the intensity of such incident light exceeds well-known levels, the diode laser may be damaged, and the reliability of the FAMP may be compromised.
Consequently, there is a need in the art for a FAMP design which reduces the intensity of fiber laser pump light incident on the diode laser.
Yet another problem of this FAMP design is related to unwanted reflections of signal light. These reflections affect the performance of a FAMP via a phenomenon known as multipath interference (MPI); that is, imperfect WDMs allow a portion of the signal light to be coupled to the fiber laser where it is reflected back toward the WDM. The reflected signal light is coupled through the WDM and, after being amplified by the gain section of the FAMP, reaches the output of the amplifier out of phase with the original signal light. The out-of-phase component raises the noise floor of the system.
Therefore, there is also a need in the FAMP art to reduce signal reflections which produce MPI.